The invention relates to an optical information medium for erasable recording using a laser-light beam, the medium including a substrate carrying a stack of layers includes a first and a second carbide layer, a recording layer of a phase-change material arranged between the carbide layers, for recording amorphous bits when in its crystalline state, and a first dielectric layer arranged between the substrate and the first carbide layer.
The invention also relates to the use of such an optical recording medium in high storage density and high data rate applications.
Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information.
The main problems in high speed phase-change optical recording related to the required capability for a large number of overwrite cycles (cyclability), i.e. the number of repeated writing (amorphization) and erasing (recrystallization) operations, and a proper crystallization speed. A high crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD-RAM, DVD-ReWritable, DVR (Digital Video Recorder) and optical tape, where the complete crystallization time (complete erase time: CET) has to be shorter than 50 ns. If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) of the previous recording cannot be completely removed (recrystallized) during DOW. This will cause a high noise level.
An optical information medium of the type mentioned in the opening paragraph is described in co-pending U.S. patent application 08/993,133 filed Dec. 18, 1997. The described medium of the phase-change type includes a substrate carrying a stack of layers including a first dielectric layer of e.g. (ZnS).sub.80 (SiO.sub.2).sub.20, a phase change recording layer of e.g. a GeSbTe compound, which recording layer is sandwiched between two relatively thin carbide layers of e.g. SiC, a second dielectric layer, and a reflective layer. Such a stack of layers can be referred to as an I.sub.1 I.sup.+ PI.sup.+ I.sub.2 M structure, wherein M represents a reflective or mirror layer, I.sub.1 and I.sub.2 represent the first and second dielectric layer respectively, I.sup.+ represents a carbide layer, and P represents a phase-change recording layer. With such a stack, a CET value of 25 to 30 ns is reported. A CET value of 28 ns corresponds to a user data bit rate (UDBR) of 35 Mbit/s, or a data bit rate (DBR) of 41 Mbit/s. UDBR is DBR corrected for overhead, i.e. data used for addressing codes and error correction.
The above citations are hereby incorporated in whole by reference.